Exploring Our Solar System and Its Origin

2. Exploring Our Solar System and Its Origin

3. Sun • Over 99.9% of solar system’s mass • Made mostly of H/He gas (plasma) • Converts 4 million tons of mass into energy each second

4. Mercury • made of metal and rock; large iron core • desolate, cratered; long, tall, steep cliffs • very hot and very cold: 425°C (day), –170°C (night)

5. Venus • nearly identical in size to Earth; surface hidden by thick clouds • hellish conditions due to an extreme greenhouse effect: • even hotter than Mercury: 470°C, both day and night • atmospheric pressure equiv. to pressure 1 km deep in oceans • no oxygen, no water, … • perhaps more than any other planet, makes us ask: how did it end up so different from Earth?

6. Earth and Moon to scale Earth and Moon to scale • Earth • An oasis of life • The only surface liquid water in the solar system; about 3/4 of surface covered by water • A surprisingly large moon

7. Mars • Looks almost Earth-like, but don’t go without a spacesuit! • Giant volcanoes, a huge canyon, polar caps, more… • Water flowed in the distant past; could there have been life?

8. Jupiter • Much farther from Sun than inner 4 planets (more than twice Mars distance) • Also very different in composition: mostly H/He; no solid surface. • Gigantic for a planet: 300  Earth mass; >1,000  Earth volume. • Many moons, rings… Great Red Spot

9. SATURN • Giant and gaseous like Jupiter • most spectacular rings of the 4 jovian planets • many moons, including cloud-covered Titan • currently under study by the Cassini spacecraft

10. Uranus • much smaller than Jupiter or Saturn, but still much larger than Earth • made of H/He gas, and hydrogen compounds (H2O, NH3, CH4) • extreme axis tilt — nearly tipped on its “side” — makes extreme seasons during its 84-year orbit. • moons also tipped in their orbits…

11. Neptune • Very similar to Uranus (but much smaller axis tilt) • Many moons, including unusual Triton: orbits “backward”; and is larger than Pluto. Wispy white clouds are thought to be crystals of methane.

12. Pluto • A “misfit” among the planets: far from Sun like large jovian planets, but much smaller than any terrestrial planet. • Comet-like composition (ices, rock) and orbit (eccentric, inclined to ecliptic plane, long -- 248 years). • Its moon Charon is half Pluto’s size in diameter • Best current photo above;

13. New Horizons mission launch Jan 2006, arrival at Pluto in 2015…

14. Asteroids *100,000+ rocky objects within the orbit of Jupiter *Also called minor planets *The largest, Ceres, has a diameter of about 900 km or ~ (560 mi) *Orbit the Sun in the same direction as the planets *Most orbit the Sun at distances of 2 to 3.5 AU, in the asteroid belt

15. TNOs - Trans-Neptunian Objects *1,000+ small bodies orbiting beyond the orbit of Neptune *The largest of these are known as dwarf planets *Include Pluto, Eris, Charon, Makemake, etc. *Orbit the Sun in the same direction as the planets *Most orbit within the Kuiper belt at 30 AU to 50 AU

16. Comets • Objects that result when Kuiper belt objects collide • Fragments a few kilometers across, diverted into new and elongated orbits • The Sun’s radiation vaporizes ices, producing tails of gas and dust particles • Astronomers deduce composition by studying the spectra of these tails created by reflected sunlight • Oort cloud comets orbit out to 50,000 AU

17. Clues to the Formation of Our Solar System Our Goals for Learning •What features of our solar system provide clues to how it formed? • What theory best explains the features of our solar system?

18. Common Properties of Planet Orbits in Our Solar System As viewed from above, all of the planets orbit the Sun in a counter-clockwise direction. The planets orbit in nearly the same plane. All planets except Pluto have an orbital inclination of less than 7°.

19. Rocky asteroids between Mars & Jupiter Icy comets in vicinity of Neptune and beyond Asteroids and comets far outnumber the planets and their moons

20. A successful theory of solar system formation must allow for exceptions to general rules

21. Summary: Four Major Features of our Solar System

22. Classifying the Planets The planets (except Pluto) fit into two groups: The Jovians or Outer Planets: Jupiter Saturn Uranus Neptune The Terrestrials or Inner Planets: Mercury Venus Earth Mars 10

23. Terrestrial Jovians Larger mass and size low density mostly H, He, & hydrocarbon compounds No solid surface many moons rings Farther from sun and farther apart Smaller Mass and size higher density made of rock and metal Have solid surfaces few moons no rings Closer to Sun and closer together

24. Size, Mass, and Density The Jovian planets have much bigger diameters and even larger masses than the terrestrial planets.

26. Though less massive than the Jovians, Terrestrial planets are much more dense.

28. Again, with the exception of ODD BALL Pluto, the rotation rates of Jovian planets on their axes are much faster than the Terrestrial planets.

30. Despite these fast rotation rates, the diameters of the Jovian planets are tremendously larger than those of the Terrestrial Planets.

32. What theory best explains the features of our solar system?

33. According to the nebular theory our solar system formed from a giant cloud of interstellar gas (nebula = cloud)

34. The lightest and simplest elements, hydrogen and helium, are abundant in the universe. Heavier elements, such as iron and silicon, are created by thermonuclear reactions in the interiors of stars, and then ejected into space by those stars. Ejection of Matter from Stars LARGE STAR NEAR THE END OF ITS LIFE FORMATION OF PLANETARY NEBULA SUPERNOVA EXPLOSIONS

35. Great clouds of gas and dust ejected from old stars are gathered into regions from which new stars can be made. This region in the constellation of Orion shows new stars still surrounded by the nebula from which they were formed.

36. Summary of the Nebular Model for formation of the solar system.

37. Inner parts of disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice.

38. Fig 9.5 Inside the frost line: too hot for hydrogen compounds to form ices. Outside the frost line: cold enough for ices to form.

39. Why are there two types of planets? Outer planets get bigger because abundant hydrogen compounds condense to form ICES. Outer planets accrete and keep H & He gas because they’re bigger. Inner planets too hot, gases evaporate

40. Other Star Systems Forming We can look at young star systems developing today. The planets orbiting these stars are formed from the surrounding disks of gas and dust, called protoplanetary disks or proplyds.

42. PLANET FORMATION Within the disk that surrounds the protosun, solid grains collide and clump together into planetesimals. The terrestrial planets are built up by collisions and the accretion of planetesimals by gravitational attraction. The Jovian-like planets are formed by gas accretion.

43. Four Unexplained Features of our Solar System √ Why do large bodies in our solar system have orderly motions? √ Why are there two types of planets? --> 3) Where did the comets and asteroids come from? 4) How can we explain the exceptions the the ‘rules’ above?

44. Comets and asteroids are leftover planetesimals. •Asteroids are rocky because they formed inside the frostline. • Comets are icy because they formed outside the frostline

45. Four Unexplained Features of our Solar System √ Why do large bodies in our solar system have orderly motions? √ Why are there two types of planets? √ Where did the comets and asteroids come from? --> 4) How do we explain the existence of our Moon and other “exceptions to the rules”?

46. Earth’s moon was probably created when a big planetesimal slammed into the newly forming Earth Other large impacts may be responsible for other exceptions like rotation of Venus and Uranus Remember! Early in history of solar system, such impacts far more common

47. Review of nebular theory Fig 6.27

48. Four Features of our Solar System - Explained √ Why do large bodies in our solar system have orderly motions? √ Why are there two types of planets? √ Where did the comets and asteroids come from? √ How do we explain the existence of our Moon and other “exceptions to the rules”?

49. When did the planets form? We cannot find the age of a planet, but we can find the ages of the rocks that make it up We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it

50. The decay of radioactive elements into other elements is a key tool in finding the ages of rocks